For example, a phase varying apparatus has been proposed as an apparatus for controlling the opening/closing timing of an intake valve of an engine or an exhaust valve thereof. This phase varying apparatus has a structure in which a sprocket to which the driving force of a crankshaft of the engine is transmitted and a camshaft that is a component of a valve operating mechanism are rotated together. Although the sprocket and the camshaft are rotated in synchronization with each other, rotational delay occurs in a rotational drum relative to the sprocket when a braking force acts on the rotational drum by use of an electromagnetic brake means. The phase of the camshaft relative to the sprocket is varied in conjunction with the rotational delay of the rotational drum (see Patent Literature 1). This phase varying apparatus employs a structure in which engine oil is introduced into a relative sliding portion between a friction material of a clutch case and the rotational drum through an oil passage formed in the camshaft, an oil sump provided on the radially inner side of the clutch case, and an oil-introducing notch formed in a front edge part of the inner peripheral wall of the clutch case. Therefore, relative sliding surfaces of the friction material and the rotational drum can be cooled.    [Patent Literature 1] Japanese Published Unexamined Patent Application No. 2002-371814 (see pages 4 to 6, FIG. 1 to FIG. 4.)